Transmitting system



30, 194-2. J.BR|AN 2,287,862

TRANSMITTING SYSTEM Filed Dec. 24, .1940

V INVENTOR I JOSEPH BRIAN Maou/af/an ATTORNEY Patented June so, 1942 TRANSMITTING SYSTEM Joseph Brian, MontreaLQuebec, Canada, assignor to Radio Corporation of America, a corporation of Delaware I Application December 24, 1940, Serial'No. 371,519

4 Claims. (.Cl. 250-17) My present invention relates to radio transmitters. It is often desirable to transmit and receive on the same high frequency carrier frequency. To enable such duplex signaling with closely situated transmitting and receiving apparatus, it is essential to out oh the transmitted carrier while receiving and to turn it on quickly as soon as the voice or other signal wave is to be transmitted. My present invention has as its main object the provision of an improved transmitter in which the carrier is quickly turned on in the presence of a signal to be transmitted and effectively turned off upon the removal or absence of audio signal waves.

Further objects, advantages and features of my invention will appear as the more detailed description thereof proceeds. The latter will be given with the aid of the accompanying drawing which is a schematic wiring diagram of a radio transmitter in which various features of my invention have been incorporated.

Referring to the drawing, waves from a high frequency oscillation generator 802 are fed through its tuned output circuit 2 and coupling condenser i to the input grid of radio frequency amplifier 807. The latter, through tuned circuit 6 and coupling condenser 8, excites the paralleled input grids of the paralleled beam tube power amplifiers 8l3, SW. The output of the power amplifiers 8l3, BB is fed through tuned output circuit Hi to a suitable radiating antenna l2.

Modulation is accomplished by varying the screen grid and plate potentials of the power amplifier tubes 8l3, M3 in the following way: Voice or keyed tone signals are fed through transformer T50| to audio frequency amplifier A. F. A. The amplified signal is then fed through transformer T503 in push-pull to the control grids of modulator tubes V502, V503.

The plates of the modulator tubes are connected in phase opposition across the primary of transformer T104. Suitable modulation voltages are then impressed by the action of the secondaries of transformer T704 on the screen grids and plates of power amplifier tubes 813, M3 which are also supplied with suitable high positive voltage from lead 20.

Normally, in the absence of signal waves in the primary of T501, switch SI is open, breaking the supply of positive voltage to the screen grid 22 of oscillator tube 802. The open screen grid circuit causes oscillator tube 802 to block and thereby prevents the generation by, and flow of high frequency energy from, the oscillator to the power amplifier tubes 801, M3 and 8l3'.

However, with transformer T50l energized by signal waves, alternating voltages appear across the secondary of transformer T503. the latter voltage is fed through condenser C5l2 and resistor R5l'l to the right hand diode rectifler section of tube UV506.

The rectified voltage developed across the time constant'circui't consisting of resistor R516 and condenser C511 is applied to the grid of the left hand electrode section of normally blocked section of tube UV5'06. This causes current flow to take place through the formerly blocked left hand section of tube UV506 and, therefore, current flow through the coil of relay E503, quickly closing switch SI'.

Closure of switch S1 applies positive potential to the screen grid of oscillator tub'e 802, unblocking the same and causing it to generate high frequency oscillations.

Time constant circuit C5 and R5l6 is so adjusted that rectified voltage builds up thereacross practically instantaneously. This effectively closes switch SI on the first voice impulse. It is to be noted in this regard that switch SI is arranged to positively act on the first voice impulse and thereby effectively prevents clipping, although this period may be alteredif desired. The charge on the time constant circuit is permitted to leak off in about five seconds.

In the absence of signal waves, circuit R516- C5l| discharges, the left hand section of tube UV'zifit blocks, current flow through relay E503 ceases, and switch SI opens. This opens the circuit to the screen grid of oscillator tube 8t? and causes it to block and also, therefore, the

cessation of oscillation generation.

In order to adapt the oscillator for different frequency control requirements, a switch S401 is provided which may be set to the right for connecting the control grid through capacitor C4 to the output circuit for deriving feed-back energy therefrom. When switch S lill is moved to its left-hand contact, however, a piezo-electric crystal Y40l is interjected in shunt with the grid-cathode resistor R403. In this position, the oscillator becomes crystal controlled.

The anode of oscillator tube 802 is connected to the upper terminal of a tuned circuit 2 which is formed by the coil LI, and shunt condenser C3. The lower terminal of the tuned circuit is connected through inductance L401 to the positive terminal of a direct current operating A part of source. Series connected capacitors Cl and C2 are also in shunt with the inductance Ll.

It i to be noted that the common terminal between condensers Cl and C2 and also the cathode of oscillator 802 are grounded. Condenser C3 is a small condenser operated by himetallic strip B. S. in such a way as to maintain the frequency of oscillation constant despite variations in ambient temperature.

For operation under crystal control, switch S4! is thrown to its left hand position connecting crystal Y40I in shunt to the grid cathode resistor R403. Feedback in this case takes place through small feedback condenser C409 which is small relative to either condenser Cl or C2.

To break up the symmetry of the power amplifier tube circuits and to prevent generation of parasitic oscillations, the parasitic suppressor circuit, formed of inductor L2 and resistor R2, is connected as shown between the paralleled control grids of amplifier tubes 8|3, 8l3'. For obvious purposes, choke L40l and bypass condenser CMH are connected as shown to the plate voltage supply lead.

Having thus described my invention, what I claim is:

1. In combination, an oscillation generator having grid, plate and cathode electrodes connected to circuits for the production of high frequency oscillations, said generator also having a screen grid, an amplifier having a cathode, a grid and a plate, means for supplying oscillations from the generator to the grid and cathode of the amplifier, a load circuit connected to the plate and cathode of the amplifier, a source of audio frequency waves, means to amplify the audio frequency waves, means to utilize the amplified audio waves for varying the voltage on the screen grid and plate of the power amplifier to modulate the output thereof, an electromagnetically operated switch for opening and closing the direct current circuit to the screen grid of the oscillation generator, a blocked electron discharge system in series with said electromagnetically operated switch normally preventing flow of current through the coil of said switch and thereby maintaining the same open, a rectifier for rectifying a part of the amplified signal waves, a' time constant circuit across which rectified current is impressed, and a connection from said time constant circuit to said blocked electron discharge system operating to unblock said system in the presence of rectified voltage across said time constant circuit, thereby closing said switch and thereby permitting said oscillation generator to generate oscillations.

2. In a radio transmitting system, an oscillator circuit arrangement comprising a discharge tube having an input circuit connected between a cathode and a control grid thereof, and an output circuit connected between said cathode and an anode thereof, said tube also having a screen grid connected through switching means to a suitable source of screen grid potential, a circuit for amplifying modulation potentials, a mixer-amplifier circuit device arranged to be fed with energy from said oscillator and from said modulation amplifier circuit, and an electronic relay for controlling said switching means by modulation potentials derived from said modulation amplifier circuit, said relay comprising two discharge paths one of which is normally blocked and is in circuit with an actuating winding for said switching means, the other of said discharge paths being constituted as a rectifier of modulation energy, the rectified output from which serves to unblock the first mentioned path, thereby to close said switching means for supplying screen grid potential to said oscillator tube.

3. The combination according to claim 2 and including a time constant circuit connected between a cathode appropriate to the rectifier discharge path of said electronic relay and a control grid in the other discharge path of said relay.

4. The method of controlling the output from an oscillation generator of the screen grid type by opening its screen grid circuit in the absence of modulation energy and by closing said screen grid circuit in the presence of modulation energy, which method comprises rectifying a component of the modulation energy, unblocking a normally blocked discharge path in response to the rectification of said modulation energy, closing said screen grid circuit in response to the unblocking of said discharge path, and introducing an appreciable delay in the subsequent blocking of said discharge path upon cessation of the modulation energy.

JOSEPH BRIAN. 

